Silicon Nitride has generated considerable interest as a potential replacement for metals in applications requiring high strength at elevated temperatures, good thermal shock resistance and high resistance to oxidation and corrosion.
It is well known that the properties of the densified body are greatly dependent on the density, and it has been found necessary to add sintering aids to Silicon Nitride in order to fully densify the body. Typically the sintering aids used are Al.sub.2 O.sub.3, BeO, MgO, TiO.sub.2, ZrO.sub.2, HfO.sub.2, and the oxides of the Group III elements of the periodic table, Scandium, Yttrium, Lanthanum, Cerium, etc.
It is also known that these sintering aids combine with the SiO.sub.2 which is normally present in the Silicon Nitride raw material to form grain boundary phases, which can be either crystalline or amphorous. The final product thus consists of grains of Si.sub.3 N.sub.4, either in the alpha or beta crystalline form, surrounded by one or more grain boundary phases consisting of silicon, nitrogen, oxygen, and the sintering aids. The properties of the final product are greatly influenced by the composition and properties of this grain boundary phase(s).
Silicon nitride compositions for use as cutting tools have concentrated on improving the high temperature properties of strength, hardness, and oxidation resistance.
For example, U.S. Pat. No. 4,227,842 to Samanta relates to a silicon nitride cutting tool for machining cast iron having a composition containing silicon nitride, silicon oxide and yttrium oxide. This patent also mentions a Prior art cutting tool material set forth in Japanese Pat. No. 49-113803 (10-30-1974) by Kazutaka Ohgo which appears in Chemical Abstracts, Volume 84, 1976, page 286. According to this work, silicon nitride is sintered with a metal oxide spinel. The spinel is formed prior to sintering by mixing magnesium oxide and yttrium oxide and heating to the appropriate temperature.
U.S. Pat. No. 4,388,085 to Sarin, et al relates to a composite cutting tool having an intergranular refractory phase comprising silicon nitride, magnesium oxide, and silicon dioxide.
U.S. Pat. No. 4,073,845 to Buljan relates to a process for producing a silicon nitride composite from an amorphous silicon nitride powder and a sintering aid which includes magnesium oxide or yttrium oxide.
U.S. Pat. No. 4,280,973 to Moskowitz, et al relates to a process of making a silicon nitride cutting tool by cold pressing and then sintering a powder constituting at least 75% by weight silicon nitride, another powder selected from the group consisting from yttrium oxide, magnesium oxide, cerium oxide, zirconium oxide, and mixtures thereof, and an additional powder selected from the group consisting of aluminum oxide, tungsten carbide, tungsten silicide, tungsten, tantanium carbide and mixtures thereof.
U.S. Pat. No. 4,327,187 to Komatsu, et al relates to a process of making silicon nitride articles consisting of yttrium oxide, aluminum oxide, aluminum nitride, and another powder selected from the group of titanium oxide, magnesium oxide, and zirconium oxide.
It is generally accepted that Si.sub.3 N.sub.4 bodies made with MgO have inferior high temperature properties to those bodies made with Y.sub.2 O.sub.3 sintering aids. In many instances, the improvement in high temperature properties have been accompanied by the impairment of other properties. While the prior art recognizes the influence of grain boundary phases on the mechanical properties of silicon nitride compositions, the effect of grain boundary phase composition on the machining performance of silicon nitride cutting tools is incompletely understood. Notwithstanding the properties of the foregoing compositions, enhancement of the metal cutting performance of silicon nitride cutting tools remains a highly desirable object.